1. Field of the Invention
This invention relates to a transducer for converting a differential pressure to an electric signal and more particularly to an improved transducer employing a strain sensitive semiconductor resistor on a substrate having a high modulus of elasticity. This improved transducer is simple in its construction; capable of operating at low differential pressures; and capable of operating at high temperatures and exposed to nuclear radiation.
Transducers of the general type taught by this invention are well known in the art. One example of such a prior art transducer is shown in U.S. Pat. No. 3,559,488; it employs a semiconductor beam or diaphragm as the sensing element. In this prior art transducer, which is typical of the transducers of the type generally contemplated by this invention, the resistance of resistors formed in outer surface of the semiconductor beam or diaphragm changes as the beam or diaphragm changes as the beam or diaphragm deflects in response to changes in pressure. This change in resistance produces a change in current flow, and the current signal may be calibrated in terms of the pressure.
Recently, there have been proposals to make differential pressure transducers employing a strain sensitive diaphragm comprised of a sapphire substrate onto the surface of which a silicon resistance pattern is formed. Such a combination has certain desirable qualities as a strain sensitive element particularly in environments such as high temperature environments and environments which include exposure to nuclear radiation. However, as will be appreciated by those skilled in the art, sapphire has a very high modulus of elasticity. Prior art proposals for transducer designs employing strain sensitive silicon on sapphire diaphragms have not proved entirely satisfactory particularly for low pressure applications.
The objects of this invention relate to a design for a transducer employing a silicon on sapphire strain sensitive element for use in low differential pressure applications. Specifically, the objects include a design which permits; (a) a relatively large movement of a pressure responsive element at low differential pressures; (b) simple overload protection; and (c) realistic manufacturing tolerances.
Briefly, this invention contemplates a transducer in which a short sapphire slab carries a silicon strain sensitive resistive pattern. The sapphire slab is an element of a much larger beam, one end of which is embedded in a header. A fluid filled capsule connected to the other end of the beam moves as the relative pressure of the fill fluid and the external pressure changes. As the beam moves, it changes the stress at the outer surfaces of the sapphire slab and varies the resistance of the silicon resistor to produce an electric signal.